#include <bits/stdc++.h>
using namespace std;
#define PI 3.1415926

int main()
{
	double xa,xb, ya,yb;
	char c1,c2,c3;
  int h,r;
	float f;
	scanf("%lf%lf%lf%lf",  &xa, &ya, &xb, &yb);
  printf("%.3lf", sqrt((xa-xb)*(xa-xb) + (ya-yb)*(ya-yb)));

  return 0;
}
